Devices such as an optical modulator and an optical switch utilize nonlinear optical effects, and among these particularly an electro-optical effect that is a change in refractive index in response to an electric field. Conventionally, as nonlinear optical materials exhibiting this effect, inorganic materials such as lithium niobate and potassium dihydrogen phosphate have been widely used. To satisfy requirements such as higher nonlinear optical performance and production cost reduction, organic nonlinear optical materials are attracting attention and studies for the practical use thereof are actively carried out.
There are growing expectations for development of polymer materials having extremely excellent electro-optical properties compared to those of conventional inorganic materials to fabricate a super high-speed modulation device or to implement a low power consumption device technology, in particular. These polymer materials cause organic nonlinear optical compounds to uniformly disperse in a polymer matrix or to bind to polymer side chains, and orient the compound molecules to give rise to the electro-optical properties. As such a nonlinear optical compound, a push-pull compound having a π-conjugated chain, one end of which is electron donative and the other end of which is electron attractive, is used.
Examples of a known method for production thereof include a method of applying a polymer material onto a substrate having an oriented film on a surface thereof to use the substrate orientation of the oriented film, and an electric field poling method in which a polymer material heated to near the glass transition temperature or higher is oriented by applying voltage thereto with a pair of electrodes or corona discharge in the air. Among these, the electric field poling method is preferred from the viewpoint of simplicity of apparatus and the high degree of orientation of nonlinear optical compounds.
An optical waveguide required when a nonlinear optical material is used for a light propagation-type device is formed in a layered structure, in which a polymer core portion containing a nonlinear optical compound and cladding portions having a refractive index smaller than that of the core portion on and under or around the core portion are formed. As the nonlinearity of the nonlinear optical compound contained in the core portion becomes higher, the electrical resistivity of the core portion tends to become lower. Accordingly, the electrical resistivity of the cladding portions tends to be relatively high compared to that of the core portion, and consequently, voltage cannot be efficiently applied to the core portion and appropriate electro-optical properties cannot be obtained.
To solve this issue, Patent Document 1 describes a method in which a polymer compound having an alkylammonium group is added into a cladding material to reduce the resistance value of the cladding portion and thus improve the poling efficiency.